Will Avast block the FBI Virus that locks all computer files and demands $%$ to unlock?
yes all AV does…
but the problem is that the bad guys release new versions every day to avoid detection :-\ as they do with all malware they create
here you can see new versions added every day http://www.avast.com/en-eu/virus-monitor / http://www.avast.com/en-eu/virus-update-history
Kaspersky has an automatic ransomware protection against these guys. But these guys are detected fast normally. Everything what is callf lockscreen in virus update history is ransomware in some way.